Fluorinated ionomers are widely used in electrochemical cells such as fuel cells and chloralkali cells, as well as in fluid drying and humidification. In physical form they may be films or tubing. The forms are fabricated by melt extrusion or by extrusion or casting of solutions of the ionomers or precursors thereof. In the case of melt extrusion, melt-processible precursors of the ionomers are used because the ionomers themselves are difficult to melt-process. For solution processing, solutions of ionomer precursor can be made in fluorocarbon solvents, or the ionomers themselves may be put into solution, for example by the process disclosed in U.S. Pat. No. 4,433,082, to Grot.
Melt extrusion is limited in how thin the film can be made. One reason for this is that the glass transition temperature (Tg) of most ionomer precursor polymers is near room temperature or below. Films of less than 1-2 mils (25-50 μm) in thickness have a Saran-like clinging property and require use of interleaving during film windup. The thin film attracts contamination and stretches, that is, lacks dimensional stability.
A further drawback of thin extruded films is that they are difficult to hydrolyze. Hydrolysis is necessary to convert the ionomer precursor polymer to the ionomeric form. This generally requires caustic, water, and often solvent, and heating. The polymer swells when hydrolyzed and thin films are difficult to handle in this condition without risk of dissolution or tearing.
Because of these problems, ionomer films are generally not extruded at less than 1 mil (25 μm) thickness. Continuous hydrolysis of a 1 mil film on a commercial scale is not economical and such films are typically laminated to other films or substrates before hydrolysis. If extruded films are to be hydrolyzed without further fabrication, they must be at least 2 mils (50 μm) and more usually 3 mils (75 μm) thick.
Binary blends of fluoropolymers with fluorinated ionomer to give compositions that can be made into a fluorinated ionomer membrane that has increased strength by virtue of the presence of the fluoropolymer are disclosed in European Patent No. 345964. These blends are used in electrolysis of brine to make caustic and chlorine, i.e. in chloralkali applications. Chloride ion rejection by the membrane is improved by the blending, but membrane ohmic resistance is increased as shown by the increase in voltage of the electrolytic cell. U.S. Pat. No. 6,495,209 discloses blends of fluoropolymer with fluorinated ionomer for improving physical properties of fuel cell membranes. Blends of fluorinated ionomer with THV (tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride) copolymer are disclosed in U.S. Pat. No. 6,277,512.
Further improvements in fluorinated ionomer membrane conductivity and tensile strength are needed to meet the increasing demands that advancing fuel cell technology places on them.